System for correcting power factor and harmonics present on an electroduct in an active way and with high-dynamics

ABSTRACT

A wide range of electric loads, has functional concepts hat lie outside the common standards (resistive, inductive, capacitive), as, for instance: gas-discharge lamps, or electronic loads; in fact, for their own functional features, these loads, introduce some harmonic distortions on the absorbed current, that cannot be solved with normal compensating systems (as, for example, phase advancers). A disadvantage which affects the feeders and their respective generators, therefore reducing: their efficiency, the power capacitance that can be supplied, and the qualitative level of energy distribution. Our system succeeds in evaluating and compensating (for the part of energy that its power section allows), the harmonic distortion on the network, therefore improving the various parameters which characterize it. Besides, since the system can operate both as load and as generator, it can be structured so that it uses the power sections, already useful to hold other applications; and that greatly reduces the price-of-the-device/corrected-power ratio.

[0001] To give an easy description, this patent has been divided intothe three following sections:

[0002] 1. Analysis and considerations of the operational conditions of adistribution electric network.

[0003] 2. Description of the concept of operation of this method from afunctional point of view, and description of the experimental prototype.

[0004] 3. Description of the concept of operation of this method from amathematical point of view.

[0005] Analysis and Considerations of the Operational Conditions of aDistribution Electric Network.

[0006] The standard concept of PFC (Power Factoring Correction), hasbeen for years concerned with the phase shift between voltage andcurrent, since this parameter developed besides the Real Power (measuredin Watt), a Reactive Power (measured in VAR); a kind of power that,although didn't use energy, it increased the total current carried onthe power line, engaging more generators and all intermediate equipments(step up and step down transformers, etc.).

[0007] However, the problem was immediately solved, because it wascaused by a reactive component of inductive nature (see Table-A—Sec.Effect of an Inductive Load), which was easily compensated by a reactivecomponent of the same value, out of phase in inverse nature(capacitive), connected to the user end (power factor correction of thesystem).

[0008] The users with a typology of nonlinear current absorption,considerably help to modify, this “normal” functional condition, lackingin harmonic distortions (therefore easily resolvable); three classicexamples are:

[0009] Gas-discharge lamps;

[0010] Partial loads on each phase (with SCR/TRIAC controls);

[0011] power supply, generally.

[0012] Therefore, there is a group of users, connected to thedistribution network, whose real problem of correction, is not tointroduce a phase shift between voltage and current (a problem easy tosolve), but to introduce, on the network itself, a large quantity ofharmonics, which the users connected produces in intrinsic mode.

[0013] Table-A in the Sec. “Harmonic Distortions for some kind ofusers”, points out exactly some loads with this kind of problem. Thetable shows, for each one of them, both the graph of the current, andthe quantization of its more considerable harmonics (analysis in thetime-and-frequency-domain); the cases described are:

[0014] a neon glow lamp (the phenomenon is similar for all gas-dischargelamps)

[0015] a typical single-phase power pack (without PFC);

[0016] a typical three-phase power pack (without PFC).

[0017] That is, of course, a serious problem they tried to reducethrough several rules; in Europe, for instance, Rule no EN-61003 Is inforce. Its limits (under particular conditions of power/use) arereported in Table-B—Sec. Rule EN61003.

[0018] Therefore the cited rule doesn't succeed in solving the problem,but it can only reduce its extent. And, aselectronics-of-energy-conversion becomes more and more competitive(motor controls with VVVF systems, Energy-Conversion with PWMtechnique), and, consequently, the number of harmonics introduced on thenetwork rises up, the same problem expands, and the “Quality of theWaveform Supplied to the User” is more and more reduced (see TableB—Sec. “Typical graph of the voltage for distorted currents”.

[0019] Things being as they are, having checked the standard techniquesof power factor correction can only partially solve the problem, we gavebirth to this “Idea-Project”, supported by the growing spreading of:

[0020] the production of alternative electric energy of small/mediumpower, integrated on the feeding external network;

[0021] the conversion/use of energy with PWM technique systems; thereis, now, the problem (see Table-B—Sec. “Examples of optimal waveformsthat compensate the harmonic distortion In generation/use”) ofestablishing whether it is advantageous/useful, to a generator/user,“Not to Operate With Sinusoidal Currents, But With Exactly DistortedCurrents, Able to Correct the Harmonic Distortions On The Network.

[0022] Our project is able to:

[0023] supply, on the concerned distribution tracts, and with all theadvantages this produces, a “qualitatively” better energy.

[0024] give to the alternative-energy converters (working with PWMtechnique), also a correcting/improvement factor in the waveform'squality, for the power line to which they are connected;

[0025] improve the performances of the users (working with PWMtechnique), so that they are no more the cause of wave distortion, butof the correction/improvement of the wave itself.

[0026] Table-B—Sec. “Examples of optimal waveforms that compensate theharmonic distortion in generation/use”), describes, in a preliminary way( later on, we'll face the technical point of view), how that can berealized, in fact:

[0027] the first graph shows an half cycle of voltage, with a typicalwave's distortion, pointing out the error which characterizes it,compared with the theoretical value it should have;

[0028] the second graph shows the optimal distortion a generator shouldhave, when, connected to a power line, it has to mitigate the harmonicdistortions on the power line itself. In that case, as it is describedin the graph, it should generate a small energy on the lower values ofthe sinusoid, and concentrate it on the higher amplitudes, exactly wherethe waveform is lowered;

[0029] finally, the third graph shows the optimal distortion a usershould have when, connected to a power line, it has to mitigate theharmonic distortions that are on the power line itself. In that case, asthe graph points out, it should absorb a great deal of energy on the lowvalues of the sinusoid, and reduce it on the higher amplitudes, exactlywhere the waveform is lowered;

DESCRIPTION OF THE CONCEPT OF OPERATION OF THIS METHOD FROM A FUNCTIONALPOINT OF VIEW, AND DESCRIPTION OF THE EXPERIMENTAL PROTOTYPE

[0030] This section describes how the device architecture is structured,to transform the “idea-patent” into an “industrial-product”; is implieda series of processes summed up as follows:

[0031] A. First of all, so that “a-part” of the harmonics on the networkcan be compensated in an optimal and automatic way, they have to betransformed in measurable-parameters;

[0032] B. As it is shown in Table-C, an analog-to-digital converter,followed by a FFT converter (Fast Fourier Transform), provides for thistask;

[0033] C. Now, the system transports the parameters in thefrequency-domain, and analyzes/elaborates the parameters, considering:

[0034] The harmonic corrections already carrying out;

[0035] The power that can be used as generator/user;

[0036] The value of the voltage in output/input stage (depending onwhether it works as generator or user);

[0037] and establishing which is the best functional performance, withreference to the specific condition of functional dynamics progressing;

[0038] D. After that, the device translates again the variouselaborations in the time-domain, through a conversion that uses the“anti-Fourier-transform”;

[0039] E. Last action to do, is the arrangement of the data, transferredin the “time-domain”, in signals useful to drive the PWM powerconverter, in order to realize, on the external network, the appropriateprogrammed correction.

[0040] F. Table-C besides presenting, in Sec. “Processing Sequence”, theprocess of the events/processings, as it has been described earlier, Itpresents, In Sec. “Block diagram of functional circuits”, the blockdiagram of electronic circuits, which realize the different processes.What characterizes these circuits, as Table-C points out, is:

[0041] G. A Frequency/Phase control system, in order to generatecommands properly related to the external network;

[0042] H. A VCO (Voltage Controlled Oscillator), in order to generate afrequency synchronized to the network, but of a much more high value,thus coordinating/controlling the PWM process coming into effect;

[0043] I. A PWM-Counter, followed by a Counter-PHASE, in order tocorrelat the PWM command with that of the phase, and work at highstability (low-Jitter);

[0044] J. A FFT processor, connected to a section thatelaborates/optimizes the various parameters of harmonic distortion,followed by an FFT-1 processor, completed with a command translator inPWM ( in a short time, we will analyze in detail this circuit);

[0045] K. A PWM processor which translates the command parameters intopulse amplitudes, already arranged;

[0046] L. An output buffer which boosts/arranges the right command forthe power sections.

[0047] Now, we pay attention to the circuit described at the “Point-I”,considering how it works, and its own peculiarities; besides, in thedescription, we point out the analysis that the circuit executes, whenit controls a system which produces energy; it stands to reason that theutilization of this circuit on a users that needs to be supplied withenergy (load), reverses the sum/subtraction parameters.

[0048] The block defined as “FFT/DATA PROCESSING/FFT-1/PWM” worksaccording to the Processing-Sequence described In Table-D, that we canpoint out as follows:

[0049] 1. The input voltage, sampled by an analog-to-digital converter,that is, in its turn, controlled by a PLL system (Phase Locked Loop—inorder to obtain a perfect phase lock), is transformed into a series ofdigital data, whose acquisition times are perfectly calibrated andcorrelated to the period itself.

[0050] 2. The conversion data of the point 1, are sent to a FFTprocessor (Fast-Fourier-Transform), which executes their translationfrom the “time-domain” to the “frequency-domain”.

[0051] Therefore we obtain the discrete spectrum of the network signal,indicated as Pr(k) In the diagram.

[0052] 3. Now Pr(k) undergoes the first processing in Elb.1. The aim ofthis processing, is to test “how-much” the discrete spectrum of thenetwork signal Pr(k), is different from the discrete spectrum of thetheoretical signal Pt(k).

[0053] Therefore we obtain the discrete spectrum of the error signal,indicated as Pe(k) in the diagram.

[0054] 4. This error signal (which refers to the previous p riod),indicated as Pe(k) comes to the Elb.2. Here it undergoes a correlationprocessing, realized comparing the error that is still on the network(Pe(k)), with the correction (MemB) that the device executed on thenetwork itself at the time of the sampling we are considering (i.e. theprevious period).

[0055] The Elb.2processing Is essential for the system convergencetowards a situation of optimal energetic generation. It has to executethe next final processing, “considering” that the measurement result,which undergoes the Pr(k) analysis, is not a “neutral” result, but aresult obtained “also” thanks to the energy contribution of this device,which has a parallel programming on the network, and boosts a kind ofenergy/correction, we have to consider before establishing the followingexecutive commands; thus a processing which allows to:

[0056] Connect the device in parallel to other networks, knowing not theratio between its own part of energy contribution and the total system;therefore, it can function by itself, or connected in parallel togenerators with a much more big power than its maximum. Even so, thedevice has no difficulties in quickly and accurately “converging”towards its optimal condition of generation.

[0057] Delete all the problems that the distribution of the elecrticlines involves: line resistances, greater/smaller closeness ofperturbation loads, etc.; the system, exactly because correlates what itdynamically measures with the executed corrective-action (parametersstored in the memory), is always able to make an action immediatelycorrected, without proceeding in “step of successive approximation”;thus it realizes an excellent operating dynamics.

[0058] Obviously, so that the device can properly operate, it isnecessary for it to store the command data, in order to carry out theirsubsequent evaluation; this circuit section, indicated as Mem-A & Mem-Bin the diagram, is realized with a Shift-Memory system, that stores thecommand data, and translates them of a period, supplying the Elb-2processing with them.

[0059] Therefore we obtain the discrete spectrum of the correctionsystem, Indicated as Pc(k) in the diagram.

[0060] 5. However, the Pc(k) signal is still not utiilizable as actuatorcommand to send to the FFT-1 (described afterwards); in fact, if thesystem had only these controls, it could reach operatingcircumstances/commands of “breakdowns”; and here the term “breakdown”has to be interpretated in a very wide sense, for example:

[0061] In the attempt of making the maximum harmonic correction, thesystem could give an executive command higher than its own maximumgenerable power.

[0062] In the attempt of generating the maximum available power, itcould increase the voltage of the inverter DC/AC up to supply the userwith a voltage higher than the acceptance standards.

[0063] The processing section (Elb-3), that works comparing the Pc(k)signal with the Pp(k) signal, provides for that.

[0064] The Pp(k) signal is a complex signal, which contains the whole of“limits” that the device has to fulfil in its functional cycle. Some ofthese limits are attached to the hardware (maximum generable power, themaximum energetic peak that can be supplied, etc.), some are attached tothe standards (maximum voltage, operating frequency, etc,), others tothe operating dynamics (maximum available eolian energy, maximumavailable photovoltaic energy, etc.).

[0065] Getting out from Elb-3 one finally obtains the discrete spectrumof the fulfillment signal, indicated as Pa(k) In the diagram; a spectrumwhich is also acquired by “Mem-A” to update the memory “Mem-B” towardsthe subsequent processings.

[0066] 6. The discrete spectrum of the Pa(k) signal, Is now sent to theFFT-1 processor (Inverse-Fast-Fourier-Transform), which takes us backfrom the “frequency-domain” to the “time-domain”. Therefore, we willhave a sequence of signals, getting out from the FFT-1, that,appropriately correlating with the PLL sequencies, and after a sectionof matching/development (Output Buffer), gives the corrected commandsignal, that has to be sent to the PWM power circuit (Pulse WidthModulation).

[0067] This method allows to operate:

[0068] Under a circumstance of total absence of oscillations; in fact,each period is generated by a processing which analizes it in itsoverall parameters;

[0069] In a total automatic mode, thus able to “auto-converge” towardsthe optimal operating condition, and under any circumstance of use(whether it in single programming, or connected in parallel to biggenerators);

[0070] With an excellent cost/performance ratio. It can, in fact, bestructured as Up-Grade, on any control section, thus operate onequipments already available; a great saving when one considers that thepower section is the most expensive part, of any other conversiondevice.

[0071] We Close This Section Describing the Exerimental Prototype WeHave Arranged to Test the Functioning of the System, in its OwnOperative Conception as Generator (When it Functions as User, theOperative Criterion is Reversed).

[0072] First of all, we point out, that the control/correction system“has-to-aim” to work with the higher possible dynamics; from this pointof view, the maximum possible is to trace the harmonic components of aperiod, and to make the relevant corrections, already in the subsequentperiod; and all that is certainly possible, if we structure appropriateprocessing systems As regards our prototype, whose target is only totest the system's functioning, we preferred to work on three periods,connecting period/action as it is shown in Table-D (a period of datasurvey, another of processing, and another of correction). Even underthese circumstances, we got some good results; anyway, we intend toincrease this correction dynamics, for the models of production inseries, (that regards the device's features, but it doesn't regards its'functional conception, and thus the draft of this patent).

[0073] To give prominance to the working cycle of the prototype we haveworked out, we arranged the diagram of Table-E; following it one canpoints out the various circuits and their correlations.

[0074] The upper part of th diagram, shows the power section, and, as wcan verify, it is quite similar to a normal PWM conversion section inalternating current so the elements that the diagram reports, are:

[0075] The measure transducers for the voltage and current parameters(in the alternating and continuous section);

[0076] The power section with IGBT and integrating filter;

[0077] The Drivers of command/control, for both the IGBT connected tothe positive (Driver-Up), and for the IGBT connected to the negative(Driver-Dw).

[0078] The lower part of the diagram, shows the “Control Section”; thisis the part that points out the functional conception of the device,that we can describe as follows (from now on, we use “PCC” referring tothe Processing/Control Computer):

[0079] The network voltage (together with other measurements), goes Inthe “PCC”, which tests Its acceptance-parameters (limits offrequency/voltage/phase that the system can satisfy).

[0080] If the acceptance-parameters obey to the project's rule, the“PCC” activates the PLL circuit (Phase Locked Loop), being characterizedby a phase angle detector, and by a VCO (Voltage Controlled Oscillator),it generates the most convenient frequency, so that the PWM counter(Ctrl-Ck-Pwm) and the phase counter (Ctrl-Ck-phase), can go on in acompletely synchronous mode, with the frequency and the phase of thenetwork input This circuit is really Important because, theprecision/stability it reaches, is a parameter which defines the deviceperformance/quality as a whole. It's plain that, to correct, forexample, the ninth harmonic, one needs a much more steady system thanthe harmonic component that has to be corrected (In that case nine timesbigger than the fundamental).

[0081] From this PLL circuit, we get both the PWM commands(Ctrl-Ck-Pwm), and the command that Informs the MicroComputer of theStep-of-phase, that the period has reached (Ctrl-Ck-Phase).

[0082] This circuit can be structured with various functionalfrequencies, since the sole constraint it imposes, is to have “the resetof the phase counter” perfectly synchronized with “the completion of thperiod on the external network”. It's clear that the larger Is thenumber of samplings selected (both as PWM, and as Phase), the better arethe performances that the device offers. We have to take this choicecarefully, considering th circuit as a whole; In fact: Increasing thesampling (thus the PWM frequency), the power circuits lower theefficiency; on the contrary, the cost of the electronics of controlrises up, because of the advanced performances.

[0083] At the same time of the PLL section, and in a perfectlysynchronic mode, the “PCC” activates a conversion analog-to-digitalcircuit, that samples the network voltage, and stores the differentamplitudes and angles of the period in which they are obtained.

[0084] This parameter is also very important because the larger is thenumber of the measurement carried out, the less is the error of thesystem In the processings; but we still have to considerate that, thelarger is the number of the measurements, the more we have complexprocessings; so it Is necessary to have a processing faster/qualifiedsystem, thus more expensive.

[0085] The “PCC” can be structured in various ways, but-it has tooperate according to three different elaborative concepts:

[0086] It has to make the sampling of the data and memorize them,correlating them to the various phase sequencies, which they belong to;

[0087] It has to realize the processing of the FFT, of the FFT¹ and ofthe ELB-1/2/3, as it is shown in Table-D;

[0088] It has to arrange the commands of the “PWM-Command-Controller”,as Table-E shows;

[0089] A planning fact that can be realized using only a microprocessor(and make it working for the part to the various processing actions,with Interrupts/trap techniques), or assigning “parallel-architectures”assign “PCC” in modo distribuito.

[0090] In the realized prototype, we preferred to use threemicroprocessors working at the same time (as regards themass-production, we think it's necessary to resort to a “siliconfoundry”, and structure the “PCC” on “dedicated-chip”), and assign themthe processing phases as follows:

[0091] A first Microprocessor controls the conversion of the differentmeasures, the stability of the network in frequency/phase, and memorizesin a memory area the value of the network voltage, together with thphase angle with which the measure has been obtained;

[0092] A second Microprocessor elaborates the signals as Table-D shows(see the sections FFT, ELB-1, ELB-2). In that case, we let theMicroprocessor intercat with a Mathematical-Coprocessor, and using thealgorithm of the DFT (Discrete Fourier Transform), in order to highlyreduce the processing times (we believe, in that case, that for themass-production it is necessary to use a Mathematical-Coprocessorappropriately structured);

[0093] Finally, the third Microprocessor carries out the processings, asit is shown in Table-D (see the sections ELB-3, FF¹), and realizes thecommands that has to be sent to the Up/Dw/Driver. Also in that case welet the Microprocessor interact with a Mathematical-Coprocessor, inorder to reduce the processing times (and, also in that case, we believethat for the mass-production it is necessary to use aMathematical-Coprocessor appropriately structured). We also point outthat this section, in order to elaborate the commands that has to beassigned to the PWM-Driver, strictly interacts with the phase controller(Ctrl-Ck-Fase signal) and with the PWM controller (Ctrl-Ck-PWM signal).Therefore, its working is structured as follows:

[0094] After carrying out the DFT¹, the circuit knows the variousharmonic components that need to be compensated; since it has the stepsof phase of the fundamental frequency (Ctrl-Ck-Phase), it can obtain thecontribution that the various harmonic components give, for eachPWM-step activated;

[0095] Besides, correlating with the PWM reference signal (Ctrl-Ck-PWM),it can perfectly synchronize the commands and avoid any possiblephenomenon of instability(low-Jitter).

[0096] Last section is composed of the PWM-Controller and theUp/Dw/Driver, whose target is to guide the power circuits, with thecommands that the processing section assign to them.

[0097] Description of the Concept of Operation of this Method From aMathematical Point of View.

[0098] introduction

[0099] The sampling theorem says that, the strictly-limited-band-signalsare represented by their own samples, when the Nyquist condition(fsamp≧2fmax) is satisfied: the time-domain/frequency-domain duality,suggests that a similar result has to be valid for the spectrum of thelimited signals. As regards the sequencies, with a finite duration, withthis suggestion we reach an alternative spectrum representation, knownas Discrete Fourier Tranform (DFT).

[0100] The interest for this kind of tranform is due to the existance ofalgorithms that are particularly efficient for its evaluation, known asFFT algorithms (Fast Fourier Trasform); they allow us to calculate theirspectrum very quickly. In order to experiment our prototype we haveprepared a matrix whose calculations are pre-elaborated (Table-F showstheir structure); this matrix points out, according to the followinginput data:

[0101] The number of the samples per period (Num-Samp);

[0102] The number of the quantization levels (ADC);

[0103] The number of the harmonic that has to be calculated(N).

[0104] The system realizes the following three processing steps:

[0105] A first step to evaluate the real and imaginary components(box-real and box-imag of the Table-F).

[0106] A second step to realize the sum of the real and imaginary partsobtained(box-Sreal and box-Simag of Table- F).

[0107] A third step, with which we obtain the amplitude of eachharmonic, from the Sreal and Simag values obtained.

[0108] It is, as one can see, a estremely quick method, which is able tosupply the new correction data, exactly when the analized period ends;therefore, it is a control dynamics that can be realized in “real-time”,exeeding the scheme proposed in Table-D, where the period sequencieswere: one of measurement, another of processing, and another ofcorrection. And this is a further remarkable advantage of the dynamicssystem and of the control/correction it offers.

1) An electric system able to correct, in active mode and with highdynamics, the power factor and the harmonics on an electroduct,characterized by: 1.1 A sampler of the input voltage, made up of ananalog-to-digital converter; 1.2 A PLL circuit (Phase Locked Loop),that, as the samplings proceed, supplies the phase value of the periodunder examination; 1.3 A storage system which couples the varioussamplings with the phase value acquired by the PL circuit; 1.4 Anelectronic processing, that, using the FFT(Fast Fourier Transform),operates, on the signal under consideration, the translation from the“time-domain” to the “frequency-domain.” This processing generates thenetwork signal Pr(k) (discrete spectrum of the network signal). 1.5 Anelectronic processing, that, acquiring the frequency of the sampledsignal, fixes/establishes “how-much”, the discrete spectrum of thenetwork signal Pr(k), is different from the theoretical discretespectrum Pt(k); from this difference it obtains the error signal Pe(k).1.6 An electronic processing that, from the error signal on the networkPe(k), and from the correction the system is already realizing (Mem-B),obtains the new correction signal Pc(k). This processing is essentialfor the convergence of the system towards a situation of optimal energygeneration; in fact, it doesn't considerate the result of measurement,which undergoes the pr(k) analysis, not as a “neutral” value, but as avalue obtained “also” as energetic contribution that the system isalready operating (since it also operates on the distribution networkwhere it is connected): 1.7 an electroni processing that, acquiring thePc(k) correction system verify whether it is compatible with the Pp(k)signal, a complex signal, which contains the whole of “limits” that thedevice has to fulfil in its functional cycle; some of these limits aredue to the hardware (as: maximum generable power, maximum energetic peakthat can be supplied), others are due to the standards (as: maximumvoltage, operating frequency), and others to the operating dynamics ofthe project From this processing we obtain the discrete spectrum ofexecution signal Pa(k). 1.8 An electronic processing that, from thePa(k) signal, realizes the FFT-1, and the parameters from the“frequency-domain” to the “time-domain”. 1.9 An electronic processingthat, correlating the various amplitudes of the harmonic to correct, andthe various phase dynamic that, develop on the period (furnished by thePLL circuit), obtains the different steps of “executive command” thathas to be sent to the power circuit PWM (Pulse Width Modulation). 2)Application of an electric system according to the claim 1) on aElectric-Generator, characterized by this electric system, inserted onthe Electric-Generator, in its turn connected to the distributionnetwork, elaborates and generates, analyzing the voltage waveform on thenetwork, a waveform of distorted/dephased electric current (comparedwith the theoric sinusoidal wave of reference), that is able tocompensate (for its energetic part to which it can contribute), theharmonic distortions on the network; therefore, an action that canoptimize the quality of the distributed electric energy. 3) Applicationof an electric system according to the claim 1) on a Electric-User,characterized by this electric system, inserted on the Electric-User, inits turn connected to the distribution network, elaborates and absorbs,analyzing the voltage waveform on the network, a waveform ofdistorted/dephased electric current (compared with the theoricsinusoidal wave of reference), that Is able to compensate (for itsenergetic part to which it can contribute), the harmonic distortions onthe network; therefore, an action that can optimize the quality of thedistributed electric energy.